With a stoic grace, Binyam Mohamed has described his return to the UK today after seven long years of detention in Guantánamo Bay as "more in sadness than in anger".

I have met that sense of emptiness and loss before. I have worked with two constituents who returned to the UK after a long incarceration in Guantánamo. In both cases the men had been illegally taken, in the process known as extraordinary rendition, by the CIA from an African country; and in both cases there were allegations of torture and degrading treatment. The journey to rebuild a life and to reconnect with family has been long, slow and fraught with pain. How do you come to terms with the lost years, the shame of allegations you cannot refute, or to witnessing humanity at its very darkest? Binyam Mohamed will need his friends and family around him, and the time and space to move on. It will therefore fall to others to ask the vital questions he is too weary to ask for himself.

We must not be squeamish or turn a blind eye to what has happened to him. Over seven years he has been shackled and blindfolded, flown to dark prisons across the world and kept incommunicado. He has made allegations of systematic torture, and says he had up to 20 or 30 cuts made into his penis and genitalia, with chemicals poured on the wounds for extra pain. In Guantánamo, reports suggest he was routinely humiliated and abused, resulting in long periods on hunger strike in protest. In all this time, Mohamed was never charged with a crime.

We might have expected the government to protect a UK resident from such barbaric treatment. Instead, their fingerprints are all over his case file.

Torture is wrong, pure and simple. Civilised and democratic governments, including Britain, should have absolutely no role in a practice that is both ineffective and inhumane, and there is no excuse to put our so-called special relationship with the US before the rule of law. It is not enough to simply speak out against torture: the foreign secretary has a duty to help root out and end such practices.

We cannot stamp out torture unless we know why and how it was allowed to happen in the first place. Barack Obama's commitment to close Guantánamo is a huge leap forward, but we need a full investigation to make sure that such fundamental basic principles can never be flouted again. Without this openness and transparency, the damage done by Guantánamo will linger on long after the detention camp is closed.

The Labour government should be standing up to the United States, not colluding in a cover-up. If British residents have been subjected to torture, and if our own government have turned a blind eye, then we have a right to know. If the British government is sitting on vital evidence then it should immediately release it to the public.

Binyam Mohamed has said that, when he asked a camp guard why he was being tortured, the guard replied, "It's just to degrade you, so when you leave here, you'll have the scars and you'll never forget."

We should not forget either. The wounds and scars inflicted on Mohamed are not just a personal tragedy for him, they also represent a vicious assault on the values and humanity of our country. Labour's already bruised and battered human rights record lies in tatters. President Obama has promised a fresh start but, before the slate can be wiped clean, we have to be told the truth.

Sarah Teather is Liberal Democrat MP for Brent East and chair of the all-party parliamentary group on Guantánamo Bay